A recent semiconductor memory device has been equipped with an on-chip back bias generator in order to improve its performance and reduce the number of its exterior pins. The performance of semiconductor memory device formed in a P-type semiconductor substrate can be improved by the application of the negative voltage (usually less than -2 volts) to the semiconductor substrate, whereby the threshold voltage of a n-channel MOS transistor formed in the substrate can be stabilized, and not only an increase in operating speed but also a reduction of leakage current can be achieved due to the decrease of the junction capacitance.
However, the aforementioned improvement of performance will be secured only in the case where the variation of power supply voltage provides the back bias voltage controlled within a desired value. As a matter of fact, the power supply voltage delivered from the exterior power supply circuit of the semiconductor memory device instantaneously varies due to the unstable operation thereof and the introduction of noise. Accordingly, by the variation of such power supply voltage, the back bias generator exerts a vital influence on the on-chip semiconductor circuit. That is, when the back bias voltage falls in a large amount due to the variation of the power supply voltage, the reverse bias voltage at N+/P junction regions in the substrate will increase to easily cause a reverse breakdown. Also, when the back bias voltage rises over the ground level voltage, the junction regions will be forwardly biased and, as a result, a semiconductor memory device will cause the malfunction as a whole.